The decomposition of refuse in a landfill produces landfill gas which may include, for example, about fifty (50%) percent methane. Landfill gas within the landfill is at some positive pressure, i.e., pressure above ambient, and accordingly, the landfill gas tends to migrate vertically toward the surface of the landfill due to the differential pressure between the interior of the landfill and the atmosphere. Initially, landfill gas escaping from the surface of a landfill was considered hazardous because of its methane content. However, more recently, landfill gas has been recovered and its nonmethane components removed to varying degrees to permit it to be used as an energy source.
The common technique for recovering landfill gas for ultimate utilization is to drill a vertical well, the depth of which may be, for example, from thirty (30) feet to about 250 feet. The well is lined with a perforated casing so that landfill gas can enter the well through the perforations. To induce landfill gas to flow into the well, the well is maintained at below local landfill pressure.
When landfill gas is to be recovered for ultimate utilization, it is important to exclude oxygen from the landfill because oxygen is detrimental to the production of methane by the landfill. Accordingly, it is important that the pressure within the well not be allowed to get so low that ambient air is drawn into the landfill through the surface of the landfill. This limitation on the negative pressure within the well limits the zone of influence which each vertical well has, and this is discussed, for example, in Johnson et al U.S. Pat. No. 4,026,355.
The traditional vertical well has a number of significant disadvantages. For example, if breakthrough, i.e., the drawing of oxygen into the landfill, occurs, there may be a significant time lag between the occurrence of breakthrough and its discovery. Also, when breakthrough occurs, it may be difficult to find the source of the breakthrough at the surface. Boring of vertical wells can be difficult particularly under wet conditions, and the depth of vertical wells is a significant hazard. Leachate tends to collect in the bottom of vertical wells and must be pumped out, and access to the lower regions of the vertical well is difficult to obtain. Finally, the lateral movement or shearing of adjacent planes in the landfill can cause the well to fail in shear.